How can I help my avocado plant?

Hi everyone; I am new at posting so I don't know if I am doing this right. I am growing an avocado plant indoors and all was well however, now that it has reached 3ft the leaves are drying up. The plant is in a well ventilated and sunny spot. I make sure is well watered it has plenty of room in the pot but the leaves are drying. I don't know what to do to help it.

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